Oil Wetting and Carbon Dioxide Corrosion Inhibition of Carbon Steel with Ferric Corrosion Products Deposits

Wettability of carbon steel with corrosion product films (iron carbonate [FeCO₃], FeCO₃ with oxidized surface, and rust [FeO(OH)]) was investigated through contact angle and inhibitor performance tests. Two corrosion inhibitors, an oleic imidazoline compound (OI) and a phosphate ester compound (PE), were used. The inhibitor performance was studied in carbon dioxide (CO₂) corrosion tests at 60°C, 1 bar CO₂, 3 wt% sodium chloride (NaCl), and 20 vol% oil, where the samples were alternately exposed to oil and aqueous phase. A refined, low aromatic oil was used in the tests. Addition of both inhibitors lead to significant changes in the wettability of oxidized FeCO₃ surfaces. The contact angle tests showed that a transition from a preferential water-wet state to a preferentially oil-wet state was achieved for both inhibitors. Electrochemical measurements indicated that addition of OI enabled the oxidized FeCO₃ surface to retain an oil film after immersion in oil. Electrochemical impedance spectra (EIS) measurements revealed a significant increase in the measured, uncompen-sated resistance at high frequencies after immersion in oil. The apparent retained oil film caused a significant drop in corrosion rate in aerated solutions. The reduction in corrosion rate remained stable even after a return to a CO₂-saturated solution. A similar effect was also seen on a rusting carbon steel surface. No significant lasting effect of oil exposure was seen for PE in the inhibitor performance tests.